1. Field of the Invention
The present invention relates to acetal resin compositions overcoming the disadvantages of formation of excessive mold deposits and the smell of formaldehyde gas during mold processing and exhibiting excellent heat stability.
2. Description of Related Art
Acetal resins have been widely used as engineering resins in various fields, especially in electrical appliance and electronics industries, for their excellent mechanical and molding properties. Acetal resins, however, are easily decomposed by heating during mold processing to generate formaldehyde gas, which causes a deterioration of working environment. Moreover, the generated formaldehyde and volatile materials come from additives such as the stabilizer contained in the acetal resin, which separates out on the surface of a mold to form mold deposits. Since the mold deposits considerably deteriorate the surface of the moldings and decrease the accuracy in the dimension of the moldings, molds have to be disassembled to remove the mold deposits from their surface. This requires a great deal of labor and time so that productivity is reduced. Thus reduction of mold deposits has been strongly desired.
For the purpose of solving the above problems, there have been many proposals. Japanese Patent Publication No. 5440/1959 and U.S. Pat. No. 3,960,984 disclose acetal resin compositions containing polyamide and amide oligomers, respectively in order to reduce the amount of formaldehyde produced by the thermal decomposition. Japanese Patent Application Laid-Open No. 14859/1991 discloses an acetal resin composition comprising antioxidants having a sterically hindered phenolic structure, metal salts of carboxylic acids, and quaternary polyamides 6/66/610/12. These acetal resin compositions do not succeed in sufficiently controlling the bad smell of formaldehyde and the formation of mold deposits during the mold processing, since the polyacetals are not stabilized sufficiently.
Japanese Patent Application Laid-Open No. 50948/1990 discloses an acetal resin composition comprising the hydrotalcite of an ion adsorbent in order to improve molding strain (warpage) and cycle rate. U.S. Pat. No. 4,837,400 discloses acetal resin compositions containing hindered phenol type antioxidants, alkaline earth metal silicates and/or alkaline earth metal glycerophosphates whose heat stability is improved. The inventions described in the Japanese Patent Application Laid-Open and U.S. Patent, however, do not aim at reducing mold deposits so that the disclosed acetal resin compositions do not show the effects of the present invention.
As described above, conventional acetal resin compositions produce excessive molding deposits and formaldehyde gas in the molding process, and do not show a very good heat stability.